1. Field of the Invention
The invention regards a system for indicating the positions of movable objects, which enables for a population of movable objects or stations to keep trace of one another and for a central station or a number of central stations, in some cases, to keep trace of the positions of individual and/or all the members of that population.
2. Description of the Related Art
Systems for the determination of position are well-known since a very long time, e.g. the DECCA system, the LORAN system, the VLF OMEGA system, NDB, VOR and DME. For the use of central stations, there are known different types of radar systems, such as primary radar (PSR) and secondary surveillance radar (SSR). The SSR radar system includes a ground based interrogator and SSR Transponders in the movable objects.
In recent times has been created the American GPS (Global Positioning SYSTEM) and a Russian corresponding system called GLONASS, which both build on time signals sent from satellites having known orbit elements. When at least four such satellites are above the horizon, it is possible to obtain locally an unequivocal position with a very high accuracy. Devices for obtaining such position information are well-known and commercially available, making it superfluous here to describe them in more detail than is necessary for the description of the present invention.
The fact that a precise position can be obtained with equipment having a reasonable price has led to its proposed use for navigating and surveillance for e.g. aviation, the congestion of which has become notorious in recent years. According to GB-A-2 155 720, it has been proposed to include in an aircraft Transponder response to an interrogation from a control station, not only its usual identification response, but also the position determined by GPS of the movable object carrying the transponder.
In U.S. Pat. No. 4,835,537, hereby included by reference, it is proposed to obtain an anti-collision system where aircraft, runways, fixed obstacles and land vehicles on airports signal their positions to all participants. Thereby, the signaling is made in a single radio channel for all participants, each participant seeking a moment when the channel is unoccupied and starting signalling its message after the lapse of a random wait time. The transmitting power is held at a low level in order not to exceed an air volume radius of a monitored airspace.
At present, the responsibility to monitor and control the air traffic around the earth is divided into a number of Area Control Centers (ACC) or Flight Information Centers (FIC), each of them having responsibility over dedicated regions (Flight Information Regions--FIRs) within which ground monitoring and control of air traffic operating in certain or all parts of the airspace and/or to provide certain services to aircraft which have filed a flight plan. Each time an aircraft leaves one of those FIRs, the control center of the next FIR is notified about the trajectory taken, so that the aircraft can be detected or identified. Before entering into the next FIR the crew of the aircraft is ordered to establish radio contact with the control center which FIR the aircraft is about to enter.
The air traffic must follow a limited number of air corridors, often called "airways", or predetermined routes. The airways are established along ground based radio navigation beacons or by a combination of the locations of such radio navigation beacons and "fixed points" determined by the distance and bearing to/from such ground based beacons. The aircraft are navigating by means of receivers on board which can detect the signals from the ground based radio navigation beacons and has a display unit which shows the location of the aircraft in relation to the ground based beacon. Over and in the vicinity of land also the control centers are monitoring and controlling the movements of aircraft by radar. The minimum lateral separation between aircraft flying at the same altitude in these radar controlled areas can vary between normally 5-10 nautical miles. On final approach to an airport, the minimum radar separation between aircraft is typically 3 nautical miles. In areas without radar coverage, the minimum separation for air traffic at the same altitude and track is normally 10 minutes, which, depending on the speed of the aircraft, corresponds to a distance of up to 80-90 nautical miles.
Air traffic over the oceans follows predetermined routes. Due to the fact that it is not possible to determine an aircraft position by means of ground based radio beacons or by radar when operating over the oceans, the separation between aircraft must be significantly increased. Normally, the minimum lateral separation between aircraft flying on the same track and at the same altitude is, as in the case over the North Atlantic, 60 nautical miles. At present, the number of airways and routes is limited, and aircraft have to be confined to so-called "slots", which are apportioned to the air companies or to the individual aircraft, often very long in advance. Should an aircraft by any reason be delayed for a few minutes on the ground, and miss its slot, this may cause additional delays of up to several hours before the aircraft can obtain a new free slot.
Special flow management or flow control units are established in many areas of the world. The need for these flow management units and the slot allocation procedures is caused by the lack of capacity in the airways system, along the flight routes and at the busy airports.
At airports, the capacity is limited by a number of different factors. Basically, the number of runways, taxiways and aircraft gates, meteorological conditions, navigation and landing equipment, air traffic control procedures, etc., are important elements which separately or collectively affect the capacity. In situations with poor visibility, the capacity problems are increasing, mainly due to the inability of present technology to provide air traffic control with the capability to monitor and control aircraft and ground vehicle movements. The runway occupancy times are also increasing as aircraft have to taxi at lower speeds due to difficulties to navigate on the ground in poor visibility. This also reduces the airport capacity.
The number of incidents and fatal accidents caused by potential or actual collisions between aircraft on the ground and aircraft and ground vehicles has increased over the last decades. The number of such serious incidents reported in the USA is in the order of 5-7 per 100,000 take-off and landings. A number of fatal accidents have occured over the years.
A system which can offer the possibilities for air traffic control to monitor and control aircraft ground operations as well as ground vehicle movements, having each member in the system transmitting its position and identification, presenting that information presented to air traffic control, e.g. on a display, as well as providing the same information to the pilots, and which information could also be used to guide the aircraft movements would be of significant importance to the improvement of airport capacity and safety.
The capacity and safety problems are expected to become worse as air traffic is forecasted to continue to grow at a relatively high rate. A doubling of the present number of air passengers (approx. 1.2 billion) and an almost proportional increase thereto of the number of aircraft operations is expected over the next 10-12 years.
According to a spokesman for the German Aerospace Research Establishment, "If we could get one additional landing per hour at Frankfurt, over a year it would mean 10 million deutschmarks savings in delays." (New scientist 16 November 1991 p. 23).
Several international working groups have been established to review the present situation and to recommend actions to be taken to improve the capacity of the present air navigation an air traffic control system, as well as for the improvement of the safety on the ground. Several of these working groups are conducting their work under the auspices of the International Civil Aviation Orgainzation--ICAO. In several countries, especially in the USA, significant efforts are being made to solve the safety problems related to ground operations at airports.
Thus, it is globally recognized that there is a shortage of capacity and that significant economic savings are possible if systems which can provide an improved capability to control and monitor the air traffic can improve both safety and capacity.